Compressor motors and fan motors in air-conditioning applications and drive motors in electric automobile applications require extensive control of speed variation, reduction in electric power consumption, and improved maintenability. A sensor-less drive method is increasing its application as a solution to such requirements. A sensor-less drive method executes field-oriented control of a permanent magnet motor without dedicated sensors such as hall IC for detecting rotor positioning.
For instance, fan motors used in exterior units of air conditioners experience abnormal rotation in which the motor is locked unrotatably by external force or by internal disorders, and in some instances, experience significant reduction in rotational speed by reasons such as step-out. A sensor-less drive method, lacking a sensor for detecting rotor position, fails to directly detect such rotational abnormalities. To address such shortcomings, JP 2003-319698 A discloses a method of determining occurrence of abnormal rotation during position estimation. More specifically, the disclosed method evaluates the difference between the command angular frequency and the rotor angular frequency detected by zero-convergence of d-axis induced voltage.
The above described patent document describes a determiner for determining occurrence of rotational abnormalities during position estimation. The determiner determines occurrence of rotational abnormalities by referencing the estimated motor rotational speed or by induced voltage produced by a position estimator. The problem with such determiner was that extensive time is expended for detecting rotational abnormalities when precise estimate cannot be obtained or when sizable error occurs in estimate calculation.